Backflow prevention assemblies are well-known in the art. Examples of the prior art can be seen in U.S. Pat. Nos. 2,586,942 to M. H. Grove; 2,646,064 to G. A. Colton; 2,646,816 to D. G. Griswold; 2,706,488 to G. H. Harrington; 3,173,439 to D. G. Griswold et al.; 3,276,465 to F. K. Wykoff; 3,283,772 to H. W. Ensign and 3,478,778 to A. C. Curtiss et al.
These assemblies are principally used for preventing contamination of public water distribution systems by preventing backflow or back-siphonage into the system. These assemblies are usually installed, with a gate valve on either side of it, in the line between the main supply line and a service line to a particular installation, such as a hotel, factory, institution, or the like. In normal operation, the gate valves at the ends of the assembly are maintained in wide open condition, so they are not included in this invention, and are not shown herein.
More specifically, the invention relates to certain improvements in backflow prevention devices of the type including a first check valve, a second check valve arranged in series with the first check valve and a pressure differential operated relief valve connected with a zone between the outlet of the first check valve and the inlet of the second check valve for draining that zone, when a given pressure differential exists between the inlet pressure of the first valve and the pressure in said zone. The relief valve is responsive to the differential of pressures across the first check valve derived from a sensing tap-off located at the inlet side of the first check valve and communicated to one of the pressure chambers in the relief valve, while the other pressure chamber is exposed to the pressure in the aforesaid zone between the check valves.
In a "reduced pressure principle", backflow prevention assembly, the relief valve is designed so that it will open when the pressure in the zone approaches the pressure on the inlet side of said first check valve, as is well understood in the art. The accurate sensing of pressure is particularly important where, as here, the relief valve is designed to operate on a pressure differential of about 2 1/2 p.s.i.